The present application is directed generally to cannulas and specifically to the securement of the cannula to a hub member.
To be acceptable, processes for manufacturing cannula assemblies, which include a hub and cannula, must satisfy several requirements. For example, the process must have a low incidence of damaging the point of the cannula. Second, the process must have a low incidence of plugging the hollow passageway extending the length of the cannula. Third, the hub must hold onto the needle notwithstanding a film of lubricant such as silicone located on the needle. As will be appreciated, the film of lubricant is placed on the needle to reduce the resistance of skin to puncture by the needle. Fourth, the process must provide a cannula assembly in which the minimum force required to push the cannula out of the hub is greater than forces associated with its intended use, thereby precluding disassociation of the cannula from the hub.
The standard manufacturing process for cannula assemblies to be used in medical applications typically includes numerous steps. In a first step, the blunt end of the cannula is subjected to a grit blast to provide a roughened surface. A suitable epoxy is applied to the roughened surface, and the roughened surface is then placed into a cylindrical passage in a hub. The cylindrical passage has a number of peripherally disposed channels to contain the epoxy. The narrower sections of the passage (disposed between the peripheral channels) typically have a radius larger than that of the roughened surface of the cannula. The roughened surface permits the epoxy to form a strong bond between the peripherally disposed channels of the hub passage and the roughened end of the cannula. The epoxy is than cured by a suitable process, such as by exposure to ultraviolet light, elevated temperatures (via radiant heating or induction heating), and the like. Alternatively, the epoxy can use a one stage curing process with air curing or a two stage curing process in which the epoxy is mixed with a hardener or other reactant to cause curing of the epoxy. In another process, the hub is injection molded when the roughened surface is in the mold. Further, detail on conventional manufacturing processes is contained in U.S. Pat. Nos. 4,581,024; 4,655,764; 5,026,355; 5,215,621; and 5,207,853, each of which is incorporated herein by this reference.
These processes can have a number of drawbacks. First, there can be incompatability with the epoxies used to secure the cannula to the hub and the plastic material in the hub. Second, the need to cure some adhesives using ultraviolet curing requires a hub that is substantially transparent. This requirement imposes further substantial limitations on the types of plastics that can be used to form the hub. The use of a thermal cure of the adhesive can cause physical or chemical changes of the plastic which can undesirably alter the physical characteristics (e.g., strength) of the material. In addition, all the above processes require extra time in to manufacture a cannula assembly, which significantly increases costs.
These and other problems are addressed by the method and device configurations of the present invention. Generally, the present invention includes a cannula assembly having an interference fit between the cannula and a surface of a hub member. The interference fit has sufficient strength that adhesives are not required to secure the cannula to the hub member. In some cases, the ability to manufacture a cannula assembly without using adhesives represents an approximate 50% reduction in manufacturing costs and a significantly increased capacity for a given plant size compared to conventional manufacturing processes.
In a first embodiment of the present invention, a method for manufacturing a cannula assembly including a hub member and a cannula is provided. The method includes the steps of:
(a) forming (e.g., by grit blasting) a roughened exterior surface on a portion of the cannula to yield a roughened portion of the cannula such that the roughened portion is adjacent to an unroughened or substantially smooth (e.g., non-grit blasted) portion of the exterior surface of the cannula; and
(b) inserting the roughened portion of the cannula into a central bore of a hub (typically after the smooth portion is inserted into the central bore), wherein a radius of the smooth portion is substantially the same as a radius of the hub bore to form a line-to-line fit between the wall of the bore and the adjacent surface of the smooth portion of the cannula and the radius of the roughened portion is slightly larger than the bore radius to form an interference fit between the wall of the bore and the adjacent, roughened portion of the cannula. As used herein, a line-to-line fit refers to a relationship between the hub bore and adjacent portion of the cannula in which the outer diameter of the adjacent portion of the cannula is the same as or slightly less than the inner diameter of the bore, and an interference fit refers to a relationship between the hub bore and adjacent portion of the cannula in which the outer diameter of the adjacent portion of the cannula is greater than the inner diameter of the hub bore.
The cannula and hub member can take on any number of configurations. The cannula is typically elongated and can have any shape. The cannula is typically formed from a metal such as a stainless steel. The hub member can be in one or several pieces and can be of many differing shapes. A preferred hub member configuration is shown in U.S. Pat. Nos. 5,714,125 and 5,910,289, which are incorporated herein fully by this reference. The hub member is typically formed from a nontoxic plastic typically used for medical devices.
The forming step can be performed by any suitable process capable of roughening the outer surface of the cannula. Examples include (i) mechanical techniques such as grit blasting or contacting the outer surface with an abrasive medium; (ii) chemical techniques such as etching with an acid, and reacting the outer surface with an oxidant; and/or (iii) thermal techniques such as heating the outer surface to a softening temperature followed by deformation of the softened area to form a corrugated surface and the like.
When grit blasting is employed to roughen the outer surface of the cannula, the preferred depth of the grit blast creates an outer diameter of the grit blasted (roughened) portion of the cannula that is slightly greater than the inner diameter of the hub bore. Typically, the outer diameter of the roughened portion is at least about 0.0001 inches, more typically at least about 0.0002 inches, and most typically at least about 0.0003 inches more than the smooth portion of the cannula. The outer diameter of the roughened portion is typically no more than about 0.001 inches and more typically no more than about 0.0008 inches more than the smooth portion of the cannula. The grit blasting is typically more circumferential than conventional grit blasting processes. Preferably, the roughened (enlarged) portion of the cannula extends at least about 320xc2x0 C. around the circumference of the cannula.
In one configuration, the roughened portion of the cannula has a length and the cannula a total length. The length of the roughened portion is from about 25 to about 50% of the total length. The length of the roughened portion typically ranges from about 5 to about 50 mm and more typically from about 10 to about 25 mm. The length of the smooth portion typically ranges from about 0.25 to about 0.75 inches. The total length of the cannula typically ranges from about 0.5 to about 1 inches.
To provide a strong interference fit, the tolerance between the radii of the roughened and smooth portions on the one hand and of the hub bore on the other is typically relatively small. In one configuration, the radius of the smooth portion is at least about 5% and more typically at least about 10% less than the radius of the roughened portion. In yet another configuration, the radius of the hub bore ranges from about 75 to about 99% of the radius of the roughened portion and from about 100 to about 125% of the radius of the smooth portion.
It is preferred that the length of the roughened portion be substantially the same as the length of the hub bore. In one configuration, the length of the interface between the hub bore and the roughened portion ranges from about 50 to about 100% of a length of the roughened portion. In the configuration, the length of the roughened portion is preferably at least about 5 mm, more preferably at least about 7.5 mm, and even more preferably at least about 10 mm but not more than about 50 mm, more preferably not more than about 30 mm, and even more preferably not more than about 25 mm.
To further enhance the retention forces between the roughened portion and the adjoining hub bore, the hub in one configuration is made of an elastomeric material that elastically deforms (expands) when the roughened portion is forced into engagement with the inner wall of the bore. Preferably, the material has a flex modulus that is at least about 200,000 psi, more preferably at least about 225,000 psi, and even more preferably at least about 250,000 psi but no more than about 500,000 psi, more preferably no more than about 450,000 psi, and even more preferably no more than about 350,000 psi. Particularly preferred materials include plastics such as polycarbonate, polystyrene, polypropylene, and poly(vinyl chloride).
The manufacturing process of the present invention is not only capable of producing cannula assemblies inexpensively and at a high throughput but also satisfies the above-noted requirements for medical applications. The process can have a low incidence of damaging the point of the cannula due to the line-to-line engagement of the substantially smooth portion and the bore of the hub. Because the process does not utilize adhesives, the process cannot plug the hollow passageway extending the length of the cannula with adhesives. The process can be effective in holding lubricant coated needles and can provide a cannula assembly in which the minimum force required to push the cannula out of the hub is five pounds or more. Surprisingly, it has been discovered that the roughened portion of the cannula, elastic wall of the hub bore, and lubricant act synergistically to provide a relatively high pushout force for the cannula assembly.
In a second embodiment of the present invention, a medical device is provided that includes:
(a) a cannula having an exterior surface having a roughened portion and an unroughened portion; and
(b) a hub member having an elastic bore, at least the roughened portion of the cannula being received in the elastic bore, wherein the bore has a radius such that an interference fit exists between the roughened surface of the cannula located in the bore and the surface of the bore.
In a third embodiment, a manufacturing method for a cannula assembly is provided that includes:
(a) grit blasting a portion of the cannula such that the grit blasted portion of the cannula""s exterior surface is roughened relative to a non-grit blasted portion of the exterior surface that is substantially smooth;
(b) inserting the substantially smooth portion of the cannula into a bore of a hub member;
(c) forcing the substantially smooth portion to pass through the bore;
(d) inserting the grit blasted portion of the cannula into the hub bore; and
(e) forcing at least most of the grit blasted portion of the cannula into an interference fit with the hub bore. In one configuration, the insertion force required to bring the roughened portion into full engagement with the hub bore is at least about 5 pounds and more typically at least about 7.5 pounds.
The foregoing list of embodiments is neither complete nor exhaustive. As will be obvious to one of ordinary skill in the art, a number of other embodiments are possible that include one or more of the above-noted features. Such embodiments are considered to be included within the scope of the present invention.